Estimation of permeability function from the soil–water characteristic curve

Highlight
• A new statistical model for the prediction of kw from SWCC in the form of S is proposed.
• SWCC in the form of S is analogous to the pore-suction distribution function.
• In the proposed equation, the kw of unsaturated soil is correlated with the SWCC fitting parameters.
• Soil volume change is also incorporated in the proposed equation.
• The proposed equation is able to predict kw from a bimodal SWCC.

Because direct measurement is time-consuming and costly, the permeability function of unsaturated soil is commonly determined by estimation using the soil–water characteristic curve (SWCC). Various prediction models (i.e., indirect methods) for the permeability function have been proposed by different researchers. Mualem (1986) categorized these prediction models into three groups: empirical, macroscopic, and statistical models. Of these, the statistical model is the most rigorous and provides the most accurate results (Leong and Rahardjo 1997). In this paper, a new equation for the calculation of the permeability function is proposed in which the permeability function of unsaturated soil is correlated with the fitting parameters of SWCC. In other words, unsaturated hydraulic conductivity is computed using an electronic spreadsheet with input parameters (i.e., the fitting parameters of SWCC). The proposed equation is shown to be the general form of the equations by Marshall (1958) and Kunze et al. (1968). In the equation, suction is considered as a variable and SWCC, in the form of degree of saturation, S, is adopted as a probability function. Soil volume change is also incorporated in this equation. Lastly, the proposed equation is verified using experimental data from the literature.